Hydrogen (high-purity hydrogen) is used in a number of industrial fields such as metal heat treatment, glass melting, semiconductor manufacturing and optical fiber manufacturing. Hydrogen is also used as fuel for fuel cells.
A hydrogen manufacturing system for industrially manufacturing hydrogen is disclosed in the following Patent Document 1 for example. The hydrogen manufacturing system disclosed in Patent Document 1 includes a vaporizer, a reforming reactor and a pressure swing adsorption gas separator. The vaporizer heats and vaporizes a raw material mixture, which contains a hydrocarbon raw material such as methanol and natural gas, water and oxygen, before it is supplied to the reforming reactor. In the vaporizer, the raw material mixture is flown through the vaporizer while being heated to a predetermined temperature using heat from a high-temperature combustion gas resulted from burning fuel. The vaporized raw material mixture undergoes reforming reactions in the reforming reactor to yield a reformed gas (which contains hydrogen). In the reforming reactor, reforming catalysts promote concomitant reactions; a steam-reforming reaction (in which hydrogen is generated as a primary product from the hydrocarbon raw material and water), which is an endothermal reaction, and a partial oxidization reforming reaction in which hydrogen is generated as a primary product from hydrocarbon raw material and oxygen), which is an exothermic reaction. By adjusting a raw material mixture component ratio thereby balancing the amount of heat-absorption by the steam-reforming reaction and the amount of heat generation by the partial oxidization reforming reaction, an autothermal reforming reaction is maintained and the reaction temperature in the reforming reactor stays substantially constant.    Patent Document 1: WO2006/006479
The pressure swing adsorption gas separator adsorbs and thereby removes unnecessary components other than hydrogen contained in the reformed gas, producing hydrogen-enriched gas as a final product. The separator has an adsorption tower loaded with an adsorbent for prioritized adsorption of those unnecessary components in the reformed gas. In the pressure swing adsorption gas separator, a pressure swing adsorption gas separation method (PSA separation method) is performed for gas separation. In the PSA separation method, gas separation is achieved by repeating a cycle in the adsorption tower which includes e.g. an adsorption process, a desorption process and a regeneration process. In the adsorption process the reformed gas is introduced in the adsorption tower to allow unnecessary components in the reformed gas to be adsorbed under pressurized conditions, and then to allow resulting hydrogen-enriched gas to flow out of the adsorption tower. In the desorption process, the pressure in the adsorption tower is reduced to allow the adsorbent to desorb the unnecessary components and then a gas (offgas) which contains residual hydrogen and the unnecessary components is discharged from the adsorption tower. In the regeneration process a cleaning gas, for example, is passed through the adsorption tower whereby adsorbing capability of the adsorbent to adsorb unnecessary components is restored. Generally, the length of time for performing one cycle (cycle time) is constant in normal operation where the reforming reactor and the pressure swing adsorption gas separator are under a constant load.
The offgas discharged from the adsorption tower is supplied to the vaporizer via piping, where hydrogen gas contained in the offgas is consumed as a fuel for vaporizing the raw material mixture. Due to a characteristic of the PSA separation method, the amount and the gas concentration of the offgas discharged from the adsorption tower vary significantly over time. In the case where unnecessary components other than hydrogen are adsorbed and removed in the adsorption tower as described above, the amount (flow) of offgas discharged from the adsorption tower is relatively large and the hydrogen concentration in the offgas is relatively high in an early part of the desorption process. However, as time passes in the desorption process, the amount of offgas discharged from the adsorption tower decreases and the hydrogen concentration in the offgas also decreases. Further, in the PSA separation method, it is sometimes impossible to discharge the offgas continuously due to adsorption tower operation cycle limitations. Therefore, if the offgas from the adsorption tower is continuously supplied via the piping at an uncontrolled pace, the amount of hydrogen gas in the offgas supplied to the vaporizer changes relatively widely over time, resulting in unstable combustion in the vaporizer.
Thus, in order to reduce such a fluctuation in the amount of offgas supply or hydrogen gas supply to the vaporizer and to stabilize the state of combustion in the vaporizer, a buffer tank of a relatively large capacity is sometimes provided in the piping which connects the pressure swing adsorption gas separator with the vaporizer. In this case of providing a buffer tank, offgas discharged from the adsorption tower is first introduced into the buffer tank. In the buffer tank, hydrogen concentration of the offgas is averaged, so that the supply flow of offgas from the buffer tank has a generally constant hydrogen concentration. In order to control the flow of offgas supplied to the vaporizer, a flow control valve is provided on the downstream side of the buffer tank.
In a hydrogen manufacturing system such as described as above, the vaporizer and the reforming reactor are supplied with a constant amount of raw material mixture (the amount of supply per unit time) in normal operation where the reforming reactor and the pressure swing adsorption gas separator are under a constant load. Meanwhile, as to the control over the offgas supply to the vaporizer, the flow control valve is set to a predetermined fixed degree of opening so that an average flow of offgas coming into the buffer tank and the amount of offgas flowing out of the buffer tank are generally equal to each other. Consequently in the vaporizer, a generally constant flow of the vaporization fuel is supplied by the offgas and a stable state of combustion is maintained. Also, in the reforming reactor, the temperature inside the reforming reactor is adjusted to a predetermined level by adjusting the ratio between the steam-reforming reaction and the partial oxidization reforming reaction which are proceeding in the reforming reactor, as described earlier. Thus, under a normal operation, the above-described hydrogen manufacturing system continues to heat and vaporize the raw material mixture as well as maintains a predetermined temperature inside the reforming reactor, using only the self-supplied heat which is generated in association with the system operation.
Now, in normal operation of the above-described hydrogen manufacturing system, if a change is to be made on the amount of production of the hydrogen-enriched gas as a final product gas, the load on the reforming reactor and the load on the pressure swing adsorption gas separator need to be changed. For example, when the amount of production of the hydrogen-enriched gas is to be increased, it is necessary to gradually increase the load on the reforming reactor and the load on the pressure swing adsorption gas separator until a state of normal operation as after the load change has been reached, so the amount of raw material mixture supply to the vaporizer and to the reforming reactor is increased continuously. Since this causes continuous increase in the amount (flow) of reformed gas generated in the reforming reactor and then supplied to the pressure swing adsorption gas separator, operating conditions for the pressure swing adsorption gas separator needs to be changed. As described above, the pressure swing adsorption gas separator is designed to repeat a cycle which includes the adsorption process, the desorption process and the regeneration process. Under this design, each process in a cycle is executed in accordance with a predetermined time chart, and this cycle is executed in a predetermined cycle time. In a cycle subsequent to a load change, the flow of reformed gas supplied to the pressure swing adsorption gas separator increases; however, the ability (capacity) of the adsorption tower to hold unnecessary components on the adsorbent is substantially constant and therefore it is necessary to shorten the cycle time correspondingly to the amount of increase in the flow of reformed gas. By repeating this, the cycle time in the pressure swing adsorption gas separator is successively shortened in each cycle until a state of normal operation as after the load change has been reached.
In the case that the load is increased, a conventional control on offgas supply to the vaporizer accompanies step-wise increase in the flow of offgas to the vaporizer in synchronisation with the operation cycle of the pressure swing adsorption gas separator. Specifically, in synchronization with cycle switching in the pressure swing adsorption gas separator, the degree of opening of the flow control valve on the downstream side of the buffer tank is increased in a step-wise manner so that the amount of offgas which flows into the buffer tank during the cycle (i.e., an average flow of offgas into the buffer tank in this particular cycle) will be equal to the amount of offgas which flows out of the buffer tank during the cycle (i.e., the flow of offgas moving out of the buffer tank in this particular cycle). Such a control as the above enables to keep a material balance between the amount of offgas which flows into the buffer tank and the amount of offgas which flows out of the buffer tank when the amount of production of the final product gas (hydrogen-enriched gas) is increased, so there is no such trouble as extreme pressure drop or surge in the buffer tank.
However, under such a control, with regard to the amount of raw material mixture supply to the vaporizer and the flow of offgas, while the amount of raw material mixture supply is increased continuously, the offgas flow is increased in a stepped manner (i.e. discontinuously). Such a step-wise change in the offgas flow causes a step-wise change in the state of combustion in the vaporizer, and as a result, there is a possibility that the amount of vaporized raw material mixture will increase discontinuously. This poses a risk that the reforming reaction will not proceed smoothly in the reforming reactor and the overall operation of the hydrogen manufacturing system will be affected. In other words, there is a risk that the hydrogen manufacturing system will not operate smoothly when increasing the amount of production of the final product gas. Such a problem can also occur when decreasing the amount of production of the final product gas, and also occur when starting or stopping the operation of the hydrogen manufacturing system.